Gaseous streams, for example natural or refined gas, are usually dehydrated to control, inter alia, the water dew point of the gaseous stream and avoid hydrate or ice formation.
To dehydrate it, the gas is usually placed in contact with a hydrophilic liquid desiccant, among the most popular being chemicals of the glycol family. Triethylene glycol (TEG) is the most widely used, but monoethylene glycol (MEG), diethylene glycol (DEG) or tetraethylene glycol (T4EG) may also be used. The water-laden desiccant is then transported to a thermal regeneration apparatus in which most of the water absorbed by the desiccant is vaporized and eliminated.
However, it is well known that it is not possible to separate the water completely from the desiccant using a thermal technique at atmospheric pressure, since the desiccant degrades at a temperature below the normal boiling point. TEG, for example, boils at 285.degree. C., but the regeneration temperature is normally limited to 204.degree. C. The purity of regenerated TEG is then about 99% by weight.
For greater purity, one conventional method consists in following the thermal reconcentration step with a stripping step using a dry or low water content gas, for example a portion of the gaseous stream dehydrated by the desiccant. This type of process, described in detail in United States patent U.S. Pat. No. 3,105,748, produces a purity of about 99.9% by weight of the regenerated desiccant.
However, apart from generally resulting in a dry loss, one problem in using gas as a stripping agent is that it prevents condensation of hydrocarbons and other heavy substances absorbed by the desiccant during the dehydration step and vaporized at the same time as the water during reboiling and stripping. In particular, aromatics thus liberated cannot be recondensed and are flared or evacuated unchanged into the atmosphere.
To overcome these drawbacks, the stripping gas may be replaced by a circuit containing a condensable agent, normally a hydrocarbon or mixture of hydrocarbons. In this type of process, described in detail in United States patent U.S. Pat. No. 3,471,370, the stripping agent is recovered after condensation at the head of the thermal concentration stage and separation of the water condensed therewith.
When very high purity of regenerated desiccant is required, for example greater than 99.99% by weight for TEG, a stripping agent which is itself very dry must be employed. If, for example, the stripping agent forms part of the dehydrated gaseous stream, it has been suggested that the water content of this gas can be further reduced by contacting it with a portion of the high purity regenerated desiccant, said portion then being returned to the stripping stage. This type of process is described in detail in United States patent U.S. Pat. No. 3,867,112. If a condensable stripping agent is used, dehydration under pressure of this agent by various means has been proposed.